Electrical measurements have become quite prevalent in medical and biomedical testing, and are used for monitoring a wide variety of functions of the human body. Some of these measurements are used in electrocardiograms, electroencephalograms and electromyograms. In order to make an electrical measurement, it is necessary to have a good electrical contact with the body to be measured through an electrode which is part of a half cell and is connected to a measuring apparatus.
It is recognized that a good electrode has certain important characteristics, such as, low impedance and stability. Since the electrode operates as a half cell, the half cell potential must be as stable as possible, with no influence on the half cell by perspiration or movement of the patient. The ultimate purpose of the electrode is the faithful transmission of the physiologically-generated electrical signal from the patient's skin to the recording and/or observing apparatus.
When the heretofore known electrode is used on a human body, it is necessary to prepare the area of the skin upon which the electrode is to be positioned. Typically, the skin is first cleaned; and then it is often abraded to provide a good electrical contact surface. The abrasion removes dead skin, which may impede the proper transmittal of an electrical signal to the electrode. A suitable electrolyte, generally in a gel form, is placed on the patient's skin. The electrode is then placed on the gel, forming a half cell, so that physiologically-generated electrical signals are observable. The electrical signal is carried to an amplifier and a recording and/or observing apparatus.
A very popular electrode construction which is commonly in use is one which utilizes a silver-silver chloride-chloride ion half cell. The silver-silver chloride-chloride ion half cell is generally formed either by compressing a mixture of silver and silver chloride powders and placing the compressed mixture in contact with a suitable electrolyte, or by forming a half cell by first electrochemically converting a surface of a silver piece to a silver chloride layer and placing a silver chloride layer into contact with an electrolyte.
These known electrodes generally have performed satisfactorily in many applications. However, these electrodes have certain undesirable properties. The electrolyte which is placed in contact with a patient's skin is generally in a gel form or a paste form. The paste is messy to handle, both for the operator and for the patient. The preparation technique causes irritation to the patient, especially when the skin is abraded for a good electrical contact. When prolonged readings are to be taken, the paste tends to dry, causing the impedance to increase at the electrode skin interface, thereby changing the observed signal by virtue of the failure to make a faithful transmission of the physiologically-generated signal. Furthermore, the electrical characteristics of these electrodes vary from electrode to electrode, so that there must be matching of electrodes and the measurements are limited to an AC amplifier. A DC amplifier is desirable in taking certain measurements, and particularly for electroencephalograms. The electrodes drift as the potential of the half cell changes in an erratic manner in relation to time. This often causes errors in measurement of signals and thereby gives a distorted view of the physiological signal-generating organ or body portion. A further problem which accompanies the known electrodes is that silver chloride cannot be kept in contact with a patient's skin for any prolonged time without causing irritation due to silver migration. Thus, prolonged continuous readings cannot be taken without subjecting the patient to tissue injury.